1. Field of the Invention
The present invention relates to a speed change control apparatus for a toroidal CVT and, more particularly, to control of an offset amount of a roller when a speed is changed.
2. Description of the Related Art
In the related art, a variety of types of continuous variable transmissions (CVT) have been known, including the toroidal CVT. A toroidal CVT is provided with input/output discs and a roller for transmitting a power by frictional engagement between the input/output discs.
The input/output discs have an overall shape similar to a circular cone having a slope of a circular arc. The input/output discs are arranged such that their protruding central portions oppose to each other, and the cross sections in the plane at which they contact each other form a rectangle from which a semicircle has been cut out. Further, a roller makes contact with the periphery of the input disc and with the inner side of the output disc so that the rotation of a portion of the input disc removed from the axis can be transmitted at a low speed reduction ratio to the portion of the output disc close to the axis. The gear ratio can be determined by changing that contact position.
Moreover, this roller is so supported by a member called the “trunnion” so that the roller can rotate and change its position of contact with the input/output discs. The angle of the trunnion axis is the inclination (or an angle of inclined rotation) of the rotating plane of the roller with respect to the input/output discs. In the toroidal CVT, the gear ratio is determined by the inclination.
When this inclination is to be varied, moreover, the trunnion is moved in the direction perpendicular to the axis of rotation of the input/output discs. In other words, the axis of rotation of the roller is offset from the position, at which point the axis of rotation of the roller and the axis of rotation of the input/output discs intersect at a right angle. This offset amount is called the “trunnion stroke”. As a result, the roller is subjected to a force in a direction to vary the inclination to control the gear ratio.
In the toroidal CVT, therefore, the inclination or the angle of rotation of the roller supported by the trunnion determined by the trunnion axis, and the offset amount of the trunnion (i.e., the trunnion stroke) in the axial direction, are detected. Thus, the gear ratio is determined from the inclination, and the necessary position displacement (i.e., the offset amount=the trunnion stroke) of the roller in the trunnion axial direction is calculated based on the deviation between the present gear ratio and the target gear ratio. Then, the trunnion is displaced by a hydraulic actuator so that the trunnion stroke (or the offset amount) reaches the target. Thus, the offset amount is controlled based on the deviation of the inclination (or the gear ratio) from the target. Such control may be performed as an electronic feedback control, as disclosed in JP-A-08-233085 or the like.
Although trunnion stroke can be subjected to feedback control as described above, because various forces are applied to the trunnion, deformation in the roller supporting mechanism or the like may cause errors in the detected value of the trunnion stroke.